This invention relates generally to compositions and methods for forming etch resistant masks, and more particularly to negative, high energy radiation resists for semiconductor device fabrication.
A resist is an adhering layer of a material having patterned openings on a support, used as a mask for etching the support exposed by the opening in the resist. The use of light as the radiation or energy source for fabricating integrated circuits by using photoresists in the semiconductor art has been common for many years. The photoresist method of semiconductor manufacture was adequate until the advent of small geometry high-frequency devices and integrated circuits requiring the formation of patterns with line widths in the neighborhood of one micron. Although 1 micron line openings, or resolution, can be obtained from photolithography in the laboratory, such line widths are not reproducible due to diffraction problems, with a practical limit of production produced openings being in the neighborhood of 3 microns in width.
The step from the use of light to electrons as a vehicle to fabricate integrated circuits was a logical one. Theoretically, since the size of an electron is only 1/1000th the size of a quantum of light, an electron beam should produce openings with line widths 1/1000th the size of openings obtained with photo resists. However, due to electron bounce back or back-scatter from the surface supporting the resist, such small width openings are not obtainable, only 1000 Angstroms being the practical lower limit size.
In the case of electron beam technology, an electron beam is scanned across the resist itself to form the desired pattern. The electron beam is controlled by a computer which has been fed the coordinates of the pattern as previously determined by a designer. Thus, the use of the electron beam has eliminated all the time lost in preparing the reduction of photography required to form a pattern for the resist. However, due to the pattern in the electron beam resist resulting from the scan of a very narrow electron beam, the required long exposure of the resist to the electron beam is the time draw back to the production use of electron beam resists.
Obviously, then, in addition to the characteristics required of a photoresist, such as good adhesion to many materials, good etch resistance to conventional etches, solubility in desired solvents, and thermostability, an electron beam resist must react to the electron beam radiation fast enough to allow a reasonable scan time of the electron beam. In order to bring electron beam technology into the production status, resists composed of thin polymer films that are capable of producing an image of one micron or less at very high scanning speeds of the electron beam are required.
Depending on their structures, polymeric materials tend to either degrade or cross-link when exposed to high energy radiation. Polymers which tend to degrade under radiation exposure are called positive resists. The exposed portion of the polymer always shows a higher dissolution rate to a given solvent than the unexposed portion of the same material. Polymers which tend to cross-link under radiation exposure are called negative resists. The exposed area of the polymer shows a lower dissolution rate to a given solvent than unexposed areas.
The practical implementation of direct e-beam lithography thus depends heavily on the availability of electron beam resists with high electron sensitivity and submicron resolution. Besides electron scattering, the two most serious causes for loss in resolution and pattern distortion are swelling during development and flow during baking. Previous attempts to synthesize polymers containing allyl groups from other bifunctional monomers have led to significant branching and gel cross-linking during polymerization which seriously degrades the performance of the polymer as an electron resist.
A major goal in the development of electron resists is to obtain high sensitivities, i.e., to obtain useful polymeric relief images with dosages of less than about 10.sup.-6 C/cm.sup.2. This is typically accomplished by using materials which incorporate vinyl, epoxy, or allyl functional groups into the resist polymer. However, polymers incorporating these e-beam sensitive groups always show significant swelling during developing. The small geometries of the high density pattern are thus distorted or collapsed.
COP, a copolymer of glycidyl methacrylate and ethyl acrylate, is widely used as a negative resist in e-beam lithography. This resist has a good sensitivity, but exhibits swelling during developing. Glycidyl groups in the COP are the main sites for the occurence of cross-linking during exposure, whereas the ethyl acrylate provides good film properties with some cross-linking probability. However, the glycidyl groups are not as sensitive as the vinyl group toward e-beam and ethyl acrylate units are so flexible in the copolymers, the COP bears low glass transition temperature which causes the quick diffusion of the developer into network and swelling during developing, and also causes polymer flow of the fine lines during post baking.
Polystyrene was one of the first negative e-beam resists used in lithography. Although this material exhibits excellent film properties, high resolution, and is readily available, it's slow response to e-beam exposure, about 50.times.10.sup.-6 coulombs/cm.sup.2, makes it an unpractical working resist.
Butadiene-containing polymers are also known to be e-beam sensitive materials. Their high sensitivity is believed to be due to the vinyl groups of butadiene units, which protrude from the polymer backbone. Swelling and thermal stability, however, are major problems encountered in using them as e-beam negative resists. Although styrene monomer can be incorporated into polymers to improve these properties, the swelling phenomenon is still a problem.
Poly(diallyl phthalate), on the other hand, contains benzene rings along the polymer chains. These aromatic rings are expected to give high glass transition temperature and to possess high thermal flow resistance, but the equal reactivity of two allyl groups on the same monomer tend to enhance the cross-linking reactions of the polymer during polymerization. Low molecular weight, highly branched polymers are always obtained before cross-linking prevailed, thus, these polymers would have a low speed and would show swelling during developing.
It is, therefore, an object of this invention to provide a method of forming an electron beam resist with better resolution and narrower width openings than is presently possible.
Another object of this invention is to provide a method of forming an electron beam resist that has the desirable characteristics of good polymer film properties and adhesion on many substrates, fast scanning rate, good processibility, thermal stability and dry etch capability.